The primary goal of this study is to increase understanding of the physiology of diving in North American freshwater turtles. This research will quantify how environmental hypoxia (severe < 50% oxygen saturation), and/or moderate hypoxia (75 - 50% saturation), which leads to hypoxic hypoxia, affects the cardiovascular system. Ischemic hypoxia can cause irreversible damage to tissues; the protection of brain and heart function during long-term submergence is critical to survival. Hypoxic hypoxia produces cardiac insufficiency and alters peripheral vascular resistance, and it is currently hypothesized that it results in significant changes in the regional distribution of cardiac output to the vital organs (e.g. brain, heart, liver), at the expense of non-vital tissues/organs (e.g. skeletal muscle, gastrointestinal tract). To test this hypothesis, experimental animals will be fitted with indwelling catheters and one or more surgically implanted ultrasonic flow probes, for the measurement of blood flow to specific organs/tissues during prolonged aquatic submergence. Periodic sampling from the indwelling arterial cannula will provide blood samples for analysis of physiologically important acid-base relevant ions, plasma stress hormone concentrations (primarily epinephrine and cortisol), blood gases (O2 and CO2) and pH. Overall, this research will show that systemic hypoxia, cold-induced metabolic depression, and the progressive metabolic acidosis associated with prolonged aquatic submergence results in preferential blood distribution that favors the vital organs over the 'less critical' tissue masses.